1. Field of the Invention
The present invention relates to a power supply apparatus that insulates a primary side from a secondary side by a transformer, and transmits an electric power to the secondary side by switching operation at the primary side, and to a method of controlling the power supply apparatus. For example, the present invention relates to a power supply apparatus that receives AC 100 V from an outlet with a commercial power as a supply source, performs rectification and smoothing at the primary side to convert the AC voltage into a DC voltage, and transmits an electric power to the secondary side through a switching transformer by the switching operation at the primary side.
2. Description of the Related Art
The power supply apparatus converts an AC voltage from an AC line into a DC voltage as an AC/DC converter, and applies the DC voltage to another device or a unit connected thereto. A switching system has been widely used in the AC/DC converter that converts an AC input voltage into a DC output voltage. In the power supply apparatus, there may be used a switching transformer in which an input and an output are configured in combination with a core material so as to magnetically couple a primary coil with a secondary coil. The coils are used in the switching transformer as described above, and an electrically conductive material such as a copper wire is coated with an insulating film in use. Further, the primary coil and the secondary coil are insulated from each other.
Further, the power supply apparatus may include a zero crossing circuit that outputs a timing at which an AC voltage of an AC line crosses nearly 0 V together with an output of the DC voltage. For example, the zero crossing circuit is used for such a control as to start the energization of an AC load connected to the AC line with reference to a zero crossing signal. For example, an AC motor, a heater having a heating element, an AC/DC power supply connected thereto and so on can be considered as the AC load.
In the power supply apparatus having the above-mentioned zero crossing circuit, the conversion efficiency of AC into DC is improved by reducing the power consumption of the zero crossing circuit. For that reason, when a timing made by the zero crossing signal is required for controlling the load connected to the AC line, a power supply to the zero crossing circuit starts, whereas the power supply to the zero crossing circuit stops in a state where the timing made by the zero crossing signal is unnecessary. This leads to an improvement in the conversion efficiency.
Further, Japanese Patent Application Laid-Open No. 2003-199336 discloses a device having a relay that interrupts a power supply to the zero crossing circuit from the AC line in a low power consumption state such as a standby mode or a sleep mode from the normal operation.
However, in Japanese Patent Application Laid-Open No. 2003-199336, there are required two control systems, that is, a control for interrupting the power supply to the zero crossing circuit by the relay in the low power consumption state such as the standby mode or the sleep mode, and a control of the switching operation. Accordingly, when the two systems are controlled at the same time, there arises a problem that the circuit scale becomes large, resulting in the useless control configuration.
Further, when said plural converters are connected to provide plural transformers as an auxiliary power supply, an electric power is consumed every time an output from an auxiliary coil of each transformer is smoothed. As a result, there arises a problem that the electric power is consumed by the plural transformers.